1. Field of the invention
This invention relates generally to an apparatus for measuring the thermal characteristics of fluids. Specifically, this invention relates to an apparatus for measuring the thermal oxidation tendencies of fluids, especially fuels used in liquid hydrocarbon burning engines.
2. Description of the Related Art
All major liquid fuel burning engines potentially face the problem of restrictive deposits within the fuel system due to poor fuel thermal oxidative stability. First seen as a problem in fuel systems utilized in gas turbine powered jet aircraft, the problem has now become more widespread and down to earth with the increased use of port fuel injectors in automobile engines.
Every fuel has a temperature above which deposits will begin to form on heated surfaces with which the fuel comes in contact. These deposits may remain on the surface or become suspended in the fuel to be transferred to other parts of the fuel system. The tendency of any fuel to form these deposits is increased by the dissolved oxygen from the air it contacts as well as by various impurities that might exist within the fuel. These deposits result in a number of harmful effects in an engine. Where such deposits occur they have a tendency to reduce the heat transfer properties of the fuel. A more important problem for automobile engines is the tendency of these deposits to clog the all important and often finely calibrated port fuel injectors. The importance of this problem can be seen by the rush in recent years of gasoline marketers to advertise the detergent characteristics of their fuels.
It was recognized early on that it was necessary to have a test device to rate the thermal oxidation characteristics of jet fuels so that their deposit tendencies could be determined prior to use in aircraft. The standards for accomplishing this test have for many years been defined by ASTM method D-3241 which utilizes an apparatus based upon U.S. Pat. No. 3,670,561 (Hundere patent). This application is an improvement upon the Hundere patent and utilizes its basic design as a starting point.
The earlier Hundere patent was designed specifically for testing the deposit characteristics of jet fuels and comprised a single reservoir containing fresh and tested fuels on opposites sides of a movable piston, along with a single heated test vessel, all within a closed test loop. In the Hundere patent fresh fuel was supplied to a test vessel within which a heated tubular member exposed the fuel to a known temperature for a selected period of time. A return line from this heated vessel went through both a cooling system and a constant speed metering pump which maintained the fuel flow within the system and returned the tested fuel to the fuel reservoir. A filter designed to trap deposits which remained suspended in the fuel due to the heating was located at the outlet of the heated vessel, and a differential pressure measuring device was connected across the filter to measure the pressure drop therethrough. By examining this filter and the heated tubular member for any solid deposits, the thermal stability of the fluid could be determined. The apparatus included a pressurized inert gas source whereby the entire system could be kept at a constant pressure in order to maintain an accurate flow across the heated test vessel and to prevent the heated test fuel from boiling.
While the apparatus of the Hundere patent provided a quality standard by which jet fuel could be tested and was widely utilized in the industry, it became questionable as to whether the apparatus could accurately identify the deposits that required high residence times about the heated surface for their formation. Evidence of this problem could be seen in both the apparatus itself and in the real world environment of aircraft and automobile engines. In the real world, it has been changes in the design of aircraft and automobile engines that have created the environment whereby fuels came in contact with heated surfaces for periods of time longer than the Hundere patent was designed to test. Within the apparatus itself, the problem was evidenced by the retention of solids formed only after high resident time periods within the metering pump mechanism. These solids appeared outside those parts of the apparatus where quantitative measurements were normally made.
In addition to the difficulties that the Hundere patent had with long residence times there were a number of characteristics of the apparatus itself that hindered an operator's ability to obtain consistent results.
One problem derived from the method of pumping the test fuel through the test apparatus. In the Hundere patent the circulation of the fuel was controlled by an in line metering pump and was further influenced by the constant pressure supplied by an inert gas source. Because the system pump was in line the fuel flow rate was in part determined by the viscosity of the fuel and in part by the efficiency of the pump. Because the pump was susceptible to contamination from suspended solids in the heated fuel, the flow rate could vary as much as 20% depending upon the extent to which the system pump had become clogged with deposits.
A second difficulty with the Hundere patent involved the necessity of maintaining the test fuel under the constant pressure of an inert gas source. Besides the inconvenience of the ancillary components needed to provide the constant pressure, the system required a mechanism for cutting the pressure when a fuel leak occurred. A preferable alternative would be a system that did not require the constant auxiliary background pressure of the Hundere patent and in general could execute the test under conditions that would allow only small amounts of fuel to escape in the event of a leak.
A third difficulty with the Hundere patent involved the constant attention required of the test operator. A preferable alternative would be a system that provided a greater level of automated test control and monitoring.